1. Field of the Invention
The present invention relates to a battery device including a charge and discharge control unit for detecting a voltage and abnormality of a secondary battery to control charge and discharge of the secondary battery, and more particularly, to a charge and discharge control circuit, a charge and discharge control unit, and a battery device that are capable of preventing a battery from entering an abnormal state or preventing an excessive current from flowing through a battery or an apparatus connected to the battery.
2. Description of the Related Art
FIG. 6 is a circuit diagram illustrating a related-art battery device. The related-art battery device includes a secondary battery 11, an N-channel discharge control field effect transistor 12, an N-channel charge control field effect transistor 13, a charge and discharge control circuit 14, resistors 22 and 31, a capacitor 32, and external terminals 20 and 21. The charge and discharge control circuit 14 includes a control circuit 15, an overcurrent detecting circuit 16, an overcurrent detecting terminal 19, a charge control signal output terminal 41, a discharge control signal output terminal 42, a DS terminal 45, a positive electrode power supply terminal 44, and a negative electrode power supply terminal 43. The overcurrent detecting circuit 16 includes a comparator circuit 18 and a reference voltage circuit 17.
The control circuit 15 includes resistors 504, 505, 506, 507, 518, and 528, reference voltage circuits 509, 515, and 516, comparator circuits 501, 508, 513, and 514, an oscillator circuit 502, a counter circuit 503, a logic circuit 510, a level shift circuit 511, a delay circuit 512, a logic circuit 520, and NMOS transistors 517 and 519.
Next, an operation of the related-art battery device is described. When a load is connected between the external terminals 20 and 21 and a current flows, a potential difference is generated between a negative electrode of the secondary battery 11 and the external terminal 21. This potential difference is determined based on a current amount I1 flowing between the external terminals 20 and 21, a resistance value R12 of the N-channel discharge control field effect transistor 12, and a resistance value R13 of the N-channel charge control field effect transistor 13, and is represented by I1×(R12+R13). A voltage of the overcurrent detecting terminal 19 is equal to a voltage of the external terminal 21. The comparator circuit 18 compares a voltage of the reference voltage circuit 17 with the voltage of the overcurrent detecting terminal 19. When the voltage of the overcurrent detecting terminal 19 is higher, the N-channel discharge control field effect transistor 12 is turned off for overcurrent protection. A setting value of an overcurrent detection current value is represented by IDOP, a voltage of the reference voltage circuit 17 is represented by V17, a resistance value of the N-channel discharge control field effect transistor 12 is represented by R12, and a resistance value of the N-channel charge control field effect transistor 13 is represented by R13. A voltage of the external terminal 21 as a threshold voltage for the comparator circuit 18 to output a detection signal is V17. At this time, the current flowing between the external terminals 20 and 21 is obtained by dividing the voltage of the external terminal 21 by the sum of the resistance values of the N-channel discharge control field effect transistor 12 and the N-channel charge control field effect transistor 13, and is represented by IDOP=V17/(R12+R13).
However, the related art has a problem in that, due to fluctuations in a resistance value of a charge and discharge control switch and fluctuations in the overcurrent detecting circuit, the overcurrent detection current value is low in accuracy to reduce the safety of the battery device.